EUV lithography (EUVL) is a next-generation lithography technology for 1x nm technology nodes. A reflective mask (or EUV reticle) is used in a single-exposure process to produce imaged features on a wafer. FIG. 1 illustrates a EUV reticle 100, according to a conventional design. A reflective multilayer stack 101 on a substrate 103 reflects EUV radiation at unmasked portions by Bragg interference. Masked (non-reflective) areas 105 of EUV reticle 100 are formed by etching buffer layer 107 and absorbing layer 109. Capping layer 111 is formed over the reflective multilayer stack 101 and protects it during the etching. The thickness of absorbing layer 109 ranges from 51 to 77 nm and may be obtained from the commercial market.
FIG. 2A illustrates a conventional EUVL single-exposure process and a corresponding mask shadowing effect. EUV reticle 200 is irradiated by incident EUV 201 via non-telecentric optics (not shown for illustrative convenience) and is reflected only at unmasked portions of reflective multilayer 203 to produce imaging radiation 205. Due to the non-telecentric optics, the incident EUV 201 is at an offset angle 207 (conventionally set to six degrees) to a Z-axis normal. A mask shadowing effect 209 is induced by the interaction of the off-axis illumination with the mask topography.
Adverting to FIG. 2B, the mask shadowing effect 209 varies depending on the orientation of the mask features with respect to incident EUV 201. Specifically, the imaged features on an exposed wafer indicate a printing difference (H-V print difference) between the horizontally oriented (H) features 211 and the vertically oriented (V) features 213 of EUV reticle 200 (orientation is with respect to the plane formed by the incident EUV 201 and plane normal Z; this plane is parallel to the vertical features and perpendicular to the horizontal features). The H-V print difference becomes even greater if either the offset angle 207 or the thickness of absorber layer 215 increases.
With the absorber thickness commercially available today, it is possible to compensate the H-V print difference for 1x nm technology nodes, but it does not scale well to smaller critical dimensions, especially for half-pitch values below 25 nm. Neither simple rule-based optical proximity correction (OPC) techniques nor using a thinner absorber layer maintains the printability and defectivity at beyond 1x nm technology nodes. In particular, it is difficult to compensate for the larger H-V print difference using simple rule-based OPC, and absorber layer 215 cannot be made arbitrarily thin without engendering reduced image contrast, process window, normalized image log-slope (NILS), and increased defectivity (e.g., pinholes) caused by increased residual light reflected by reflective multilayer 203 at masked portions.
A need therefore exists for methodology enabling EUV lithography for beyond 1x nm technology nodes while enhancing printability and improving defectivity, and the resulting device.